1. Field of the Invention
The present invention relates to a fluid catalytic cracking process using catalysts containing monodispersed mesoporous aluminosilicate matrix materials. The matrix materials have pore diameters between about 100 to 500 Angstroms (A) and alumina contents between about 5 to 40 wt. %. Zeolites are incorporated into the matrix materials to produce the catalyst compositions suitable for use in fluid catalytic cracking (FCC) of hydrocarbons. The catalyst compositions produce less coke and therefore more liquids.
2. Background of the Invention
Fluid catalytic cracking catalysts generally contain crystalline aluminosilicate components such as zeolites, active porous inorganic oxide components such as aluminas, inert components such as clays of the kaolin type, and inorganic oxide matrices formed from amorphous gels or sols which, on drying bind the other components together. It is desirable that the matrix be active, attrition-resistant, selective with respect to the production of liquids, and not readily deactivated by metals. Until recently, the zeolite content of FCC catalysts was low enough that the pore structure of the matrix was tailored to favor activity and selectivity over strength, or attrition resistance. However, present FCC catalysts contain more zeolitic material, as much as 60 wt. %, thus requiring that the pore structure of the matrix be more attrition-resistant while remaining active and selective.
Matrices of early FCC catalysts were simply amorphorous gel catalysts, i.e., silica-alumina or silica-magnesia. With time, new matrices based on silica sols and alumina sols were developed. Catalysts bound together with these sols did not have as desirable a matrix pore structure as that found in the order amorphorous gel catalysts; however, they were significantly more attrition resistant.
As refiners sought to increase the profitability of their FCC units by increasing the feed rate and/or adding higher molecular weight feeds, they had to increase the reactor temperature and/or the activity of the catalyst to increase the reaction rate. This combination of factors (heavier feeds, more active catalysts, higher temperatures) eventually reduces the selectivity of the reaction to naphtha and increases the yield of coke as molecules begin to undergo secondary reactions before they diffuse out of the pore system of the catalyst.
Diffusional constraints in FCC reactions lower the yield of intermediate products such as naphtha and increase the yield of less valuable stable end-products such as coke and gas.
Further, at the time catalysts needed to be more porous to prevent diffusional constraints, they also needed a higher degree of attrition resistance to handle the increasingly abrasive and hydrothermally extreme environments found in commercial units. Because of the limitations of conventional amorphorous gels and sols in balancing these two incompatible objectives, it has been suggested to employ monodispersed (single size) pore size materials as matrices. For example, U.S. Pat. Nos. 4,217,240; 4,257,874; and 4,272,409 teach the preparation of porous aluminosilicate sols having a uniform pore size and uniform particle size. These porous aluminosilicate sols are prepared at a pH of about 9 to 12 by the addition of a solution of sodium silicate and sodium aluminate to a silica or silica-alumina sol so that the sol does not gel prior to drying. The resulting material has 80% of its pore volume within 40% of the median pore diameter which can vary between 45 and 250A. While the resulting monodispersed materials of these references have advantages over the more conventional amorphorous gels and sols used as matrix materials, there is still a need in the art for further optimization of the balance between porosity and strength, especially for relatively high-content zeolite FCC catalysts.
Several patents exist on the use of mesoporous catalysts in fluid catalytic cracking. Among these is U.S. Pat. No. 4,588,702 which teaches a catalyst which has a pore volume greater than 0.4 cc/g as determined by water titration with 40-70% of all pores in the 100-1000 Angstroms diameter region, less than 35% of the pores between 20-100 Angstroms in diameter and at least 10% of all pores with diameters greater than 1000 Angstroms. The zeolite content of this catalyst lies between 8 and 25% by weight. Another zeolite-containing silica-alumina hydrogel-based catalyst with mesoporosity is disclosed in U.S. Pat. No. 4,226,743. On steaming, this catalyst has a maximum in dS/dD (where dS is the change in surface area and dD is the change in pore diameter, which will be discussed in more detail later) of 2.5 m.sup.2 /g/Angstrom at a pore diameter of 30 Angstroms. dS/dD monotonically decreases from this point to ca. 0.4 at 100 Angstroms and &lt;0.2 at 125 Angstroms.
Another series of mesoporous FCC catalysts based on silica-alumina matrices containing zeolite are taught in U.S. Pat. Nos. 4,215,015, 4,299,733, and 4,333,821. Nitrogen desorption analysis shows dV/dD (where dV is the incremental intrusion volume which will be discussed in more detail later) drops from 24 to 20.times.100 exp (-4) cc/Angstrom/g between 100 and 150 Angstroms and thereafter falls off sharply to a value less than 10.times.10 exp (-4) cc/A/g above 170 Angstroms for these materials. Also, a mesoporous silica alumina is taught in U.S. Pat. No. 4,310,441 which has more than 0.6 cc/g in pores between 20 and 600 Angstroms in diameter with less than 50% of the desorption volume between 20 and 600 Angstroms in pores with 20 to 50 Angstrom diameters. Further, this silica-alumina gel has more than 20% and less than 75% of the desorption pore volume of pores in the 20 to 600 Angstrom diameter region in pores of 50 to 200 Angstroms diameter. The silica/alumina mole ratio of this gel is between 1 and 3.
The catalysts of the present invention have zeolite contents of greater than 18%. The Si/Al ratio of matrices used to make catalysts of this invention is greater than 3. Pore volume of catalysts of this invention as determined by nitrogen adsorption at saturation are less than 0.55 cc/g for catalysts with 20% zeolite and less than 0.65 cc/g for catalysts with 40% zeolite by weight. All catalysts of this invention have more than 70% of their pore volume in the 100-1000 Angstrom region as measured by mercury intrusion when normalized with the pore volume obtained with nitrogen at saturation and its boiling point. Catalysts of this invention typically have values for dV/dD, as measured by mercury, of greater than 10.times.10 exp (-4) cc/Angstroms g above 200 Angstroms. Reasonable agreement between mercury intrusion and nitrogen desorption results have been reported in Langmuir 2,151-154 (1986). Further, the catalysts of this invention do not have a monotonic decrease in dV/dD as the pore size increases; rather they show a maximum in the dV/dD plot between 100 and 500 Angstroms. The catalysts of this invention also have a maximum in dS/dD as measured by mercury between 100 and 400 Angstroms of at least 0.18 m.sup.2 /g-Angstrom.